Tubular poles of various shapes have been used for supporting high voltage electric power transmission lines and lighting systems for several years. The poles are normally fabricated of steel plate but could be fabricated from other suitable materials, and have eight or 12 or more flat sides. The poles taper convergently toward the top and the flat sided walls of the poles are much thicker at the bottom of the pole than at the top to withstand loading.
Such poles must withstand loading caused by the weight of the transmission lines, and also caused by the wind applying horizontal forces on the lines and poles thereby tending to bow or flex the poles. Unusual loading must also be expected and planned for, such as the loading caused by ruptured transmission lines which may apply forces in many directions on the poles, but often will apply torsion forces tending to twist the pole about upright axes extending longitudinally through the pole.
It is significant to note that such tubular towers are usually used without guying, even at corners and dead-end locations in transmission lines. Also of particular significance is that such towers are used to support extremely heavy loads at extreme elevations. Poles of 110 to 125 feet in height are very common, and poles of 160 to 180 feet in height are not uncommonly used.
All of the loading on the tubular towers must be transmitted to its base in the ground. Such bases are of steel and concrete extending deep into the ground and long anchor bolts extending down through the base. The anchor bolts protrude slightly from the top of the base to connect to the pole.
In the past, it has been common practice to weld a steel base plate across the bottom of the pole with the edge of the base plate protruding outside of the periphery of the pole for receiving the anchor bolts. The entire loading on the pole is borne by the base plate which therefore must be extremely strong and tough.
However, there are practical limits to the sizes in which such base plates can be manufactured. For instance, the base plate of high quality steel must be approximately 9 inches thick and 6 to 8 feet across for towers of 110 feet to 160 feet in height, depending upon loading. Of course, base plates for higher towers must be still thicker and larger in width.
A major problem with such large and thick base plates is that they are often not homogeneous throughout. The steel must be strong and tough, and not brittle. The thicker the plate the less likely it is to be homogeneous. Of course, failure of a base plate caused by brittleness may cause collapse of a vital pole in a transmission line, and loss or outage of the transmission line can be a very dangerous and expensive situation.